The invention relates to a multispeed gearshift mechanism.
Multispeed gearshift mechanisms for bicycles, in particular in the form of multispeed gearshift mechanism hubs, are known in many embodiments. Like derailleurs, they are used to provide several selectable gear steps, or respectively gear ratios. Up to now it has been possible to realize up to twelve gear steps with such gears. However, a problem here lies in that the intervals between the individual gear steps desired in individual cases can be optimally designed at most with great constructive outlay and only for a small number of gear steps. It is furthermore disadvantageous that the known gearshift mechanisms have poor efficiency because of unfavorable tooth number conditions and planet wheels seated in sliding bearings, and that gear wheels with numerous different amounts of teeth are required for producing gearshift mechanisms corresponding to selectable gear steps.
In connection with a known multispeed gearshift mechanism hub of this type (DE 43 42 347 C1), the gearshift mechanism is constituted by three planet wheel gears, hereinafter called planetary gears for short. In this case the first and second planetary gears are provided with a common planet wheel carrier, or respectively strip, hereinafter called planet carrier for short, while the third planetary gear is designed as a differential gear, whose planet carrier is connected with the ring gear of the first planetary gear. In addition, in a first variant the ring gear of the differential gear is connected with the hub shell, while the sun wheel of the differential gear is coupled with the ring gear of the second planetary gear. Vice versa and in contrast thereto, in a second variant the sun wheel of the differential gear is connected with the hub shell, and the ring gear of the differential gear is coupled with the ring gear of the second planetary gear. A total of twelve gear steps can be realized by the combination of the gear ratios made available by the first and second planetary gears. However, poor efficiency results, since a high reactive power flow is generated because of the special type of coupling. Furthermore, structural hub sizes of undesirable size and weight occur because of the large diameter of the differential gear.
In connection with a further known multispeed gear of the species identified at the outset (DE 41 42 867 A1), two planetary gears connected in series and having a common planet carrier are provided, wherein the first planetary gear provides a gear ratio of more than one, i.e. a reduction, and the second planetary gear a gear ratio of less than one. Seven or nine gear steps can be realized, because on the driving side the actuator can be selectively connected with the ring gear or the planet carrier of the first planetary gear, and on the power take-off side the hub shell can be selectively connected with the planet carrier or the ring gear of the second planetary gear. However, in the process it is respectively necessary to shift the full driving or power take-off force, so that correspondingly stable, and therefore large and heavy coupling elements are required. In addition, the use of two shiftable ring gears adds considerably to the structural size and weight of the hub arrangement. Finally, this known gearshift mechanism is hard to shift under load.
Similar disadvantages result in connection with multispeed gears wherein these sun and planet wheels are respectively combined into a single planetary gear (DE 42 03 509 A1, EP 0 383 350 A1), although they also have several sun wheels and correspondingly stepped planet wheels.